Thermomechanical effects present a major challenge to developing a solid-state laser (SSL) for generation of high-average power (HAP) with near diffraction-limited beam quality (BQ). In particular, distortions to optical phase fronts caused by transverse temperature gradients within a SSL gain medium degrade beam quality (BQ) and render the output beam useless for many important applications. A class of SSL known as “active mirror amplifier” (AMA) has shown effective reduction of transverse temperature gradients and demonstrated generation of laser output with very good BQ. A general configuration of a laser gain medium in an active mirror (amplifier) configuration is disclosed in the prior art illustration of FIG. 1.
The AMA was first disclosed by Almasi et al. in U.S. Pat. No. 3,631,362 (1971). In the original AMA concept, a large aperture (up to 25 cm in diameter), edge-suspended, Nd-Glass disk (or slab) is pumped by flashlamps and liquid-cooled on its back face. These devices were used in a large-scale, giant pulse laser amplifier chain (rather than a laser oscillator) operating in a low-average power mode at a very low repetition rate (typically one pulse per hour). See for example, J. Abate et al., “Active Mirror: A Large-Aperture Medium Repetition Rate Nd:Glass Amplifier,” Appl. Opt., vol. 20, no. 2, 351-361 (1981) and D. C. Brown et al., “Active-Mirror Amplifier: Progress and Prospects,” IEEE J. of Quant. Electr., vol. 17., no. 9,1755-1765 (1981).
Brauch et al., in U.S. Pat. No. 5,553,088 (1996), discloses a variant of the AMA known as the “thin disk laser”. This device uses a diode-pumped gain medium disk with a small optical aperture, typically a few millimeters in diameter and 200-400 micrometers in thickness, soldered to a heat sink. See, for example, A. Giesen et al., “Scalable Concept For Diode-Pumped High-Power Lasers,” Appl. Phys. B vol. 58, 365-372 (1994). The prior art disclosed a laser oscillator using one or more of such disks made of Yb:YAG gain media placed in a stable resonator configuration. These devices demonstrated laser outputs approaching 1 kW average power and with a BQ around twelve times the diffraction limit. See, for example, C. Stewen et al., “1-kW CW Thin Disk Laser,” IEEE J. of Selected Topics in Quant. Electr., vol. 6, no. 4, 650-657 (July/August 2000).
The applicant's first co-pending patent application, U.S. Ser. No. 09/505,399, entitled “Active Mirror Amplifier System and Method for a High-Average Power Laser System”, which is hereby made a part hereof and incorporated herein by reference, discloses a new AMA concept suitable for operation at high-average power and good BQ. The invention uses a large-aperture solid-state laser gain medium disk about 2.5 mm in thickness and with a diameter typically between 5 and 15 cm, mounted on a rigid, cooled substrate, and optically pumped by semiconductor diodes. Pump power is injected into the front or back face of the disk. The disk is attached to the substrate by a hydrostatic pressure differential between the surrounding atmosphere and the gas or liquid medium in the microchannels embedded in the substrate.
The applicant's second co-pending patent application, U.S. Ser. No. 09/767,202, entitled “Side-Pumped Active Mirror Solid-State Laser for High-Average Power”, which is hereby incorporated by reference, discloses a large aperture AMA wherein optical pump radiation is injected into the peripheral edge of a gain medium disk. Side-pumping takes advantage of the long absorption path (approximately the same dimension as the disk diameter), which permits doping the disk with a reduced concentration of lasant ions and provides a corresponding reduction in required pump radiation intensity.
The applicant's third co-pending patent application, U.S. Ser. No. 09/782,788, entitled “High-Average Power Active Mirror Solid-State Laser with Multiple Subapertures”, which is hereby incorporated by reference, discloses an AMA wherein a very large optical aperture is filled by multiple AMA subapertures. This co-pending patent application also discloses an AMA with the laser gain medium disk attached to the substrate by a diffusion bond rather than by hydrostatic pressure.
The teachings of co-pending patent application Ser. No. 09/505,399, 09/767,202 and 09/782,788 provide numerous advantages over prior art solid-state lasers and allow generation of near diffraction limited laser output at very high average power from a relatively small device. In particular, analysis shows that an AMA module constructed in accordance with one or more of the above-referenced applications and using a Nd:GGG gain medium disk with a 15 cm diameter and 2.5 mm thickness can produce 15 kW of average laser power available for outcoupling with near diffraction limited BQ. See, for example, J. Vetrovec, “Active Mirror Amplifier for High-Average Power,” in SPIE vol. 4270, 2001. Co-pending application U.S. Ser. No. 09/505,399 discloses explicitly how multiple AMA modules may be used to construct a laser amplifier, especially as may be suitable for a laser configuration known as a master-oscillator—power amplifier. There are, however, many important applications that would benefit from a HAP solid-state laser oscillator producing a near diffraction-limited BQ output beam.
A laser oscillator employs a laser gain medium inside an optical resonator of suitable configuration. Photons oscillating from one end of the resonator to the other end thereof constitute electromagnetic energy which forms an intense electromagnetic field. The shape of this field is precisely dependent not only upon the photon wavelength, but also upon the mirror alignment, curvature and spacing, as well as the optical aperture and inhomogenieties of the laser gain medium. This field can assume many different cross-sectional shapes, termed transverse electromagnetic modes (TEM), but only certain modes, or a mixture of them, are useful for utilizing the laser power.
In many laser applications, the most desirable mode is the fundamental mode (i.e., TEMoo, Gaussian, or diffraction-limited mode), which also has the smallest transverse dimensions of all modes. In a laser oscillator, to enable extraction of a near diffraction-limited beam from a large-aperture gain medium it is necessary to design a resonator which supports a large size TEMoo mode under operational conditions. While laser gain elements in an AMA configuration may appear to be natural candidates for construction of a HAP SSL oscillator producing a near diffraction-limited BQ output beam, numerous challenges must be overcome, including:    1. While a large optical aperture of the AMA gain medium is essential to generation of high laser power, its advantages would be wasted if the optical resonator of the laser oscillator could not support optical TEM large enough to fill the AMA aperture;    2. To obtain good BQ, it is necessary to design a resonator having good discrimination against higher order TEM;    3. Large transverse dimensions of the AMA aperture restrict the designer to a relatively low laser gain per AMA module, which exacerbates the problem of extracting available laser power from the AMA gain medium;    4. Low laser gain may also limit the resonator outcoupling fraction which, in turn, may lead to a reduced laser beam intensity in the far field;    5. While using an array of AMA modules for successive amplification of the beam enables the desired laser gain to be obtained, this requires a resonator capable of producing large TEM size over a long propagation path;    6. Using multiple AMA modules in a laser oscillator increases alignment sensitivity, which, in turn affects stability of oscillating TEM. Some alignment issues may be alleviated by using a stable and rigid alignment platform (optical bench), however; such a platform often represents a constraint to device integration into a compact, lightweight package.    7. During the startup, the AMA gain medium experiences a rise in temperature until temperature gradients for steady-state operation are developed. This situation further aggravates mode and alignment stability.
It should be noted that such problems are far less severe in the thin disk laser of the prior art, which employs a very small aperture gain element and generates modest average power with modest BQ. This permitted using the thin disk laser gain medium in a laser oscillator with a stable optical resonator. Using such a stable resonator is entirely inappropriate for use with a large aperture AMA gain medium.